It has been shown in U.S. Pat. No. 2,650,205 that an alkaline setting resin starch reaction product can be produced that is capable of being polmerized to a water insoluble state under alkaline conditions. This patent discloses reacting resins, such as, ketone-aldehyde resins, resorcinol-aldehyde resins, and phenol-aldehyde resins, with starch granules under alkaline conditions so as to form resin starch reaction products. U.S. Pat. No. 2,650,205 teaches that control of the amount of resin and pH are critical in stabilizing paste viscosity.
There are numerous patents which disclose the production in "situ" of water resistant waterproof starch based corrugating adhesives. Among these are U.S. Pat. Nos. 2,884,389 and 2,886,541.
U.S. Pat. No. 3,294,716 teaches the addition of borax to the general phenol-aldehyde starch formula to increase corrugating speeds in application when lesser water resistance is acceptable.
A disadvantage of the alkaline curing aldehydric amylaceous adhesive compositions is their limited pot life. Pot life is the length of time, after preparing the adhesive with desired viscosity and other properties, during which the glue properties remain within a range suitable for use. The glue viscosity gradually increases with age, due to polymerization of the water-resistant agent as well as a reaction of the water-resistant agent with the amylaceous component of the glue. The thickening is often accompanied by congealing or gelling of the fluid composition. This change in composition can create operating difficulties for the manufacturer in that it may interfere with the application of the product, increase labor cost in keeping the corrugating machines clean, and limit production of the product to batches small enough to be used within its pot life.
An effective technique for controlling the rate and degree of copolymerization, and thus the viscosity and pot life, is to control the temperature used in pasting the starch components. This in turn controls the degree of dispersion of the starch or dextrin and therefore the availability of the hydroxyl group for reaction with other groups. Starch cooked at the boiling temperature of water or higher is much more reactive than starch cooked at lower temperatures. Alkalinity also has a bearing on the degree of pasting of the starch or dextrin since, in general, increases in alkalinity tend to decrease the pasting temperature.
Accordingly, researchers in the art have used sophisticated methods of altering the variables of time, temperature, alkalinity, and pH in adjusting the viscosity of the product to lengthen the pot life or working life of the product.
This is exemplified in the aforementioned Kesler, et al. U.S. Pat. No. 2,650,205, Corwin, et al. U.S. Pat. No. 2,884,389, and Langlois et al. U.S. Pat. No. 2,890,182.
U.S. Pat. No. 2,890,182 teaches a method of increasing the pot life of the alkaline-curing aldehydic amylaceous adhesives by adding an ingredient, such as ammonia, to the completed liquid adhesive composition that competes with the amylaceous material and the non-aldehydic component of the water-resistant agent for the aldehyde and thus slows the condensation responsible for the thickening and congealing of the composition.
In U.S. Pat. No. 3,802,897 issued to the co-inventors hereof there is described a water-resistant starch adhesive comprising a phenol and oxazolidine. The odorless starch adhesive possesses a long working life, increased viscosity stability, improved gel temperatures and pH characteristics, and is used with corrugated or laminated paperboard.
U.S. Pat. No. 3,839,060 (also issued to the co-inventors hereof) teaches a method of preparing "in situ" a water-resistant corrugating adhesive using starch, a phenol, an aldehyde, and an amino hydroxy compound. The adhesive has increased viscosity stability, and is further characterized in that it is essentially odorless and water-resistant. There is no release of irritating and/or odorous fumes, such as occurs in the usual formaldehyde containing corrugating systems, when the adhesive of U.S. Pat. No. 3,839,060 is used.